LU-1934 echo: echo client to work over OFD

LU-1934 echo: echo client to work over OFD

- echo client doesn't call obd_alloc/free_memmd() to underlaying
  device but implement own methods
- Use real lu_env in the prep/commitrw calls
- fill OBD_BRW_ASYNC flag for async writes

Signed-off-by: Mikhail Pershin <tappro@whamcloud.com>
Change-Id: Ic02d4c2ad08693cd47d8dff5e8a86812a1534763
Reviewed-on: http://review.whamcloud.com/4146
Tested-by: Hudson
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: wangdi <di.wang@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>